Reading and writing in a memory cell are generally accomplished using at least one bus during the read and write phases included in a clock signal. At the present time, it is usual to precede the write phase and/or the read phase by a phase in which a bus is precharged. The precharging phases are incorporated into the clock signal. A single bus will be considered for convenience. The purpose of precharging is to set the bus at a predetermined potential, usually equal to a supply potential Vcc. Writing and reading are accomplished by maintaining the predetermined bus voltage or by discharging the bus, depending on the binary state to be written to or read from a memory cell. Precharging is ordinarily accomplished by at least one field effect transistor when the bus has a low capacitive value. However, rapid precharging of a bus that has a relatively high capacitive value, for example, greater than 1 pF, requires the use of a bipolar transistor. The collector of this transistor is connected to the high supply potential Vcc and its emitter is connected to the bus. Its base is connected to Vcc and to ground through the respective drain-source leads of two MOS transistors of complementary types receiving the clock signal at their gates. Outside the precharging phase, the MOS transistor connected to ground conducts to ensure discharging of the base of the bipolar transistor. The precharging phase blocks this transistor and causes the other MOS transistor to conduct. This MOS transistor creates a current path coming from the supply potential Vcc to command the bipolar transistor to conduct. The end of the clock signal precharging phase cuts the current path and causes the other MOS transistor to conduct and to discharge the base of the bipolar transistor.
The precharging circuit described above has several disadvantages. Precharging of the bus by this circuit normally stops when the precharging voltage is sufficiently close to the supply voltage Vcc to cut off the bipolar transistor or cause it to conduct slightly. Under these conditions, the precharging time will depend mainly on the capacitive value of the bus and on the conductance of the MOS transistor driven by the clock signal to supply the base of the bipolar transistor. It is well known that a field effect transistor can have a wide range of current/voltage characteristics as a function of manufacturing conditions, with a ratio as high as 1:3. As a result, a precharging circuit in the prior art allows for no adjustment of the precharging voltage and commanding its voltage by the length of the clock signal cannot be effective because of the wide range of the characteristics of the field effect transistor causing the supply current lead of the bipolar transistor base to conduct.